The present invention is related, generally, to systems and methods for communicating information and for locating the geolocation of a remote transmitter. In particular, the present invention is related to a system for communicating from plural remote transmitters simultaneously without synchronization or polling and for determining the geolocation of specific ones of the plural transmitters.
It is well known to deploy communications systems in which a single, relatively central base unit communicates with plural, often mobile, units within its transmission range. Central dispatch systems, such as those employed by taxicabs, public services emergency personnel, and the like communicate voice and, more recently, digital data between the remote units and the central base unit. Still other present systems communicate digital data only from the remote units to the central base unit, such as "data radios" which often transmit remote sensor data from a wide geographic area to a central location. One known problem with such present systems is a need to coordinate the transmissions from the various remote units so as to avoid collisions with one another. A myriad of techniques have been used to provide such coordination or to resolve collisions. For example, in some systems, the various remote transmitters are polled by the central station in a logical or random sequence. Only the remote unit receiving the poll is then permitted to transmit during a succeeding predefined time period. In other systems, each of the remote units receives a common clocking signal and transmits to the central station at a time derived from the common clocking signal. Both these prior art schemes avoid collisions to some extent but at a cost of requiring each remote unit to be capable of both receiving and transmitting signals. Additionally, the time taken to send out polling and/or clocking signals reduces the amount of time available for the remote units to transmit.
Other prior art systems avoid collisions by assigning separate transmit frequencies for each remote unit, at an obvious cost of considerable amounts of bandwidth being occupied for systems having many remote transmitters. Still other systems permit collisions to occur and rely upon the remote transmitters to monitor the communicating frequencies to determine whether a collision has occurred and to reinitiate transmission of the message upon the detection of a collision. Again, such systems require the remote units to have the capability of both receiving and transmitting signals. In addition, such systems are sometimes known to have repeated collisions, delaying the receipt of the message contained therein, sometimes for relatively long periods of time. Finally, the number of remote transmitters is limited in such systems to a number which is dependent upon how often each of the remote units transmits and the length of each transmission.
It is also known in the prior art to geolocate a remote transmitting unit based on multiple receipts of a signal emanating from a remote transmitter. For example, plural receiving stations can receive a signal transmitted from a remote mobile unit and compare the times-of-arrival at the various receivers to determine a geolocation from which the signal was transmitted. Similarly, plural receiving stations can determine the angle-of-arrival of the signal from the remote mobile unit and by combining the different angle-of-arrival determinations at the plural stations can calculate the likely geolocation of the transmitting mobile unit. Generally, the complexity of such locating systems increases substantially with the number of remote units which must be tracked.
As in the situation for voice or data communications, if plural remote units must be tracked simultaneously, the possibility of collisions of their signals increases. As the signals, collide, it is often difficult (if not impossible) for the receiver to distinguish the location of the units sending the locating signals which have collided. To avoid collisions, some prior art locating systems have used polling, common clocking signals, individual frequencies, etc. so that the detecting receiver can unambiguously detect a locating signal. Still other systems have resorted to sending the locating signals multiple times to enhance the possibility that the central receiver has received at least one of the locating signals. This latter technique obviously relatively expensive in terms of bandwidth.
It is also been proposed for locating systems to use direct spread spectrum modulation techniques at the remote, mobile units to avoid the problems of collisions and limited bandwidth availability. In such systems, each of the remote transmitters may use a Pseudorandom Number ("PN") modulation to spread the locating signal across a wide bandwidth. Such systems are expected to meet with some success in the transmission of the locating signals by plural remote units; however, the use of such systems usually involves the design and operation of a highly complex bank of correlators which can test incoming signals for the presence of all the permitted PN codes in a very short period of time. A large number of possible PN codes usually results in a very complex and expensive receiver to detect the various possible codes. If a system uses plural receivers, as is often done to provide a relatively wide geographic range of coverage for a system, the costs of such relatively expensive receivers is multiplied by the need for plural such receivers.
Accordingly, it is an object of the present invention to provide a novel system and method for communicating simultaneously radio frequency signals from plural, geographically diverse transmitters without a common clocking signal or a polling signal.
It is another object of the present invention to provide a novel system and method for communicating signals from plural transmitters without a substantial number of collisions.
It is yet another object of the present invention to provide a novel system and method for communicating simultaneously from plural transmitters into a receiver without a bank of correlators.
It is still another object of the present invention to provide a novel system and method for determining the location of one or more of plural simultaneously transmitting remote units without a common clocking signal or polling signal.
It is a further object of the present invention to provide a novel system and method for tracking the movement of plural remote units at a relatively low cost per unit.
It is yet a further object of the present invention to provide a novel sys-em and method for communicating simultaneously from plural remote transmitters without synchronization of the units to each other of to the receiver(s).
It is still a further object of the present invention to provide a novel signal processing architecture which simultaneously tracks the location of plural transmitting units using a time invariant matched filter.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.